The research will examine the differential degree to which deficits in processing capacity and processing speed (timing and activation) contribute to the cognitive dysfunction of three neurologic populations who vary in the distribution of cortical and subcortical pathology: demented patients with Alzheimer's Disease and with Progressive Supranuclear Palsy, and both demented and non-demented patients with Parkinson's Disease. A control group of healthy elderly will be employed, and 20 subjects will be included in each group. The proposed investigation will contribute to both clinical and theoretical purposes. First, it may lead to the development of clinical instruments that are more sensitive to differences among groups of demented patients and to changes in their mental status. These instruments may facilitate the evaluation of the effects of both pharmacologic and other forms of treatment, and the prediction of the prospective nature and severity of cognitive disorders in aging patients. Second, it will also contribute to the evaluation of current theories on the underlying information-processing activation deficits in demented patients with different etiologies. The research program will be divided into three stages. In Phase I subjects will be selected on the basis of a detailed clinical history, neurological examination, measures of mental status, activities of daily living, and assessment of depression. In Phase II a battery of standardized neuropsychological tests will be used to evaluate the cognitive disorders of the patients (e.g., WAIS, WMS, naming, continuous performance, etc.). Phase III will consist of an in-depth information processing analysis of psychomotor speed (simple and choice reaction time, alerting functions, motor synchronization), language and memory (e.g., recognition memory, naming latency, semantic facilittion, etc.). The measures in Phase II and III will be readministered to appropriate subjects during a follow-up one year later.